1. Field of the Invention
The present invention relates to a waveform analysis system.
It is known in the art of waveform analysis to store a waveform of a voiceprint, electrically scanned signature or the like in a digital memory, and sequentially scan the addresses of the memory to display the waveform on a cathode ray tube screen. The memory may be a circulation shift register or a random access memory. It is desirable to provide some means of measuring the magnitude of the waveform at any selected point and also the time difference between two points. Such a system is especially useful in the analysis of aperiodic waveforms.
2. Description of the Prior Art
In a known system, an address counter is driven by clock pulses to sequentially address the memory locations for display. A digital keyboard type switch is provided to input an address of a point on the waveform at which it is desired to measure the magnitude of the waveform. A comparator compares the address input by the switch with the address in the address counter and generates a signal in response to coincidence. The signal causes a marker to be displayed on the screen at a corresponding point and also the contents of the location to be latched into an indicator for digital display of the magnitude of the waveform. This system suffers from a disadvantage in that it is not easy to guess the numerical address of the desired point on the waveform exactly, and a number of addresses must be keyed into the system in order to locate the desired address by estimating the distance between the desired point on the waveform and the marker and making address corrections based on the estimates until the marker is located on the desired point. This prior art system is therefore slow and cumbersome to operate.
It is further desirable to be able to display only a selected portion of the waveform in magnified form on the screen to facilitate analysis of minute characteristics of the waveform. With the keyboard input system of the prior art, a change in the scale of the magnified waveform on the screen will means that two points at two corresponding addresses will be spaced apart from each other by different distances depending on the magnification. This makes it even harder to guess the address of the point at which the magnitude of the waveform is to be measured, and at high magnification a wrong guess can easily place the marker off the display.
Another problem associated with the magnitude and time measurement of the prior art system is that the waveform is often stored in the memory in scaled form. For example, if the memory is capable of storing a binary number having 8 bits, the memory can store 256 discrete states (0 to 255). If a waveform to be stored in the memory has a maximum magnitude of, for example, 500 volts, it cannot be stored on a one bit per volt basis, but must be scaled down by a factor of at least two. The binary number representing each point on the waveform is thereby the actual value of the input voltage divided by the scale factor such as two. The time scale of the stored waveform is often also compressed or expanded to fit the waveform into the memory.
To provide a direct digital indication of the actual magnitude of the waveform and time difference (or space difference in the case of a signature) it is necessary to provide a descaling system for the digital displays. In prior art systems, such descaling means comprise read-only memory (ROM) converters or complicated gating systems which are expensive to implement and are limited in the descaling range which can be provided.